1. Field of Invention
The present invention relates to a drive circuit for electro-optical devices using organic electroluminescence (hereinafter “EL”). In particular, the invention relates to an enhancement in a driving method of implementing light emission with a precise level of brightness even in a low-grayscale display area.
2. Description of Related Art
A related art method of driving electro-optical devices, such as EL devices, includes an active-matrix driving method in which electro-optical devices can be driven with low power without causing crosstalk, and the durability of the electro-optical devices can be enhanced. Since EL devices emit light with a level of luminance corresponding to the magnitude of a current to be supplied, it is necessary to supply a precise value of a current to the EL devices to obtain a desired level of brightness (see, for example, International Publication No. WO98/36407).
FIG. 13 is a schematic illustrating a display apparatus based on the active-matrix driving method. In this display apparatus, as shown in FIG. 13, scanning lines Vs1 through VsN (N is the maximum number of scanning lines) and data lines Idata1 through IdataM (M is the maximum number of data lines) are disposed in a matrix in a display area to display images. A pixel circuit Pmn (1≦m≦M, 1≦n≦N) including an EL device is disposed at each intersection of the corresponding scanning line and the data line. The scanning lines Vsn are sequentially selected by scanning circuits, and a data signal corresponding to a halftone value is supplied from a D/A converter to each data line Idatam.
In the display apparatus, however, it takes time to write low-grayscale data signals, and the writing of the low-grayscale data signals may become insufficient.
In particular, the above-described problem becomes noticeable in a method of supplying a data signal having a current level associated with the grayscale. This method is referred to as a “current program method”. Since the value of a program current supplied to a data line corresponds to the grayscale to be displayed by a pixel (dot), the amount of current flowing in the data line becomes extremely small for a low grayscale image. With a small value of current, it takes time to charge and discharge the parasitic capacitance of a data line, thereby prolonging the time required to program a predetermined value of current in a pixel circuit. It is thus difficult to complete the data writing during a predetermined writing period (in general, during one horizontal scanning period). As a result, as the light-emission efficiency of EL devices is increased, the program current becomes even smaller, which makes it difficult to program a precise value of current in a pixel circuit.
Additionally, the current value in a low-grayscale display area is a few tens of nA or smaller, which is close to a leak current value of a transistor. Accordingly, the influence of a leak current on a program current cannot be negligible so as to decrease the S/N ratio, thereby lowering the sharpness in the low-grayscale display area of a display apparatus.
Moreover, as the resolution of a display is increased, the number of data lines becomes larger. Accordingly, the number of data lines to connect a pixel matrix substrate and an external driver controller is increased, which makes it difficult to connect the driver controller with the pixel matrix substrate due to a decreased pitch of the data lines. This increases the manufacturing cost of the display apparatus.